1. Field of the Invention
This patent application relates to techniques for acquiring images from a solid-state imager when exposure to the scene is controlled by either an asynchronous lighting strobe, or by the asynchronous opening of a shutter. The techniques that we describe do not require an electronic connection between the strobe/shutter and the sensor in order to work, and are hence applicable for use in systems where there is a physical reason, or an electronic reason, why this connection is not feasible.
2. Description of Related Art
Solid state image sensors dominate electronic imaging applications such as CCTV, video cameras and camcorders, and scanners, and are the basis of newly developed markets such as PC-cameras for videoconferancing, medical vision, machine vision and Digital Stills Cameras.
One popular form of image sensor is the Charge Coupled Device (CCD), whilst sensors built entirely within standard CMOS processes are also gaining currency. Roth have their relative merits when applied to these techniques.
As used herein the expression “asynchronous stimulus” means a stimulus the timing of whose occurrence is not known in advance and which stimulus is associated with the presentation of an image to be captured to the solid state image sensor. As discussed herein various kinds of solid state image sensors known in the art may be used in the present invention, including CCD sensors as well as sensors such as those disclosed in our earlier patent publication WO91/04633, in which, following a resetting of the sensing cells, charge is built up on the sensing cells in response to incident radiation impinging thereon and the built up charge subsequently converted into a voltage signal during an integration period, and this cycle repeated upon the next resetting of the sensing cells.
In some systems it is desirable to separate the operation of the sensor from the exposure mechanism. One such application is Electronic Film, for use in conventional Silver Halide Cameras such as 35 mm SLR (Single Lens Reflex). Here the solid state sensor replaces the chemical film within the camera, and as with chemical film the exposure is controlled by the shutter of the camera. In order that such an Electronic Film can work without user modification of the camera to access the shutter control signal, or with older non-electronic cameras, it is necessary for the sensor to auto-detect that it has been exposed. This system must offer a high probability of successful detection, and be scene independent, working under the widest possible range of camera exposures, and additional operating conditions such as flash and fill-in flash.
Another application is in medical vision and in machine vision, where exposure/illumination occurs through an illumination strobe, and there are physical or electronic reasons why a synchronisation pulse between the light source and the sensor cannot occur. For example it may be necessary to isolate the light source from the detector for reasons of safety, as in an X-ray system.
FIG. 1 shows a conventional general imaging system incorporating a solid state image sensor 1 (incorporating an array of sensing cells) with a shutter 2 (electronic, mechanical or electromechanical), a lighting strobe 3, and a detector 4. The imaging system also includes strobe/shutter control means 5, and sensor timing and detector control means 6. There is no timing interaction between the strobe/shutter control means 5, and the sensor timing and detector control means 6.
The shutter 2 and/or the lighting strobe 3 provide means of asynchronous stimulation of the image sensor 1 in order to capture an image of an object 7. The classic approach to the problem would be try to detect the asynchronous event and to then subsequently instigate an exposure and acquisition sequence for the image sensor. The problem with this approach is that it puts design pressure on achieving an asynchronous event detector that is sufficiently fast and reliable that the interaction between activating the image sensor and the asynchronous stimulus does not corrupt the effective exposure. FIG. 2 shows a timing diagram of an image acquisition sequence commonly used with the imaging system of FIG. 1, where the detector triggers the release from reset of the array of the sensor 1, putting it into integration. The array is then read when the stimulus has gone away. In this example the solid state image sensor 1 and the detector 4 see the stimulus simultaneously, as in the case of a lighting strobe 3. As can be seen the time for the detector to trigger, Td, reduces the effective amount of the stimulus, Ts, to an amount Te, that is approximately equal to:—Te=Ts−Td 
If there is a spatial distance between the detector 4 and the image plane of the solid state sensor 1 with respect to the stimulus, as in the case of a blade shutter 2 in an SLR camera, then the detector trigger time Td can result in a gradient of exposure across the array of the sensor 1. FIG. 2b shows an example of what would happen to an array if the detector 4 was located to the left hand side of the array, and the shutter 2 was opening from the right hand side of the array. If Tsh1 is the time the shutter takes to cross the array and Tsh2 is the subsequent time for the shutter to pass from the array to the detector, then as the diagram shows the two sides of the array see different effective stimuli, Te1 and Te2, as defined by:—Te1=Ts−Td−Tsh1−Tsh2Te2=Ts−Td−Tsh2
This problem can be reduced by using detectors on the side of the shutter that opens first, but still if the time to detect Td is greater than the time to reach the array Tsh2, then there will be a gradient of exposures across the array. The effective stimuli will be somewhere between the following values:—(Ts−Tsh)<Te<(Ts−Td), where Tsh2>Td dependant on the position in the array. This is clearly undesirable.